A person seeking to buy glasses usually has to go in person to an optometrist or an eyewear store and try-on several glasses frames to see if they fit them. Typically this requires a few hours of browsing through several rows of glasses frames and trying on many pairs of glasses frames, most of the time without prior knowledge of whether a particular glasses frame is suited to the person.
Allowing people to virtually try on glasses frames would greatly improve the efficiency of trying on glasses frames. However, it would be desirable for the placement of the glasses frames in the virtual try-on experience to be accurate in order to better approximate the try-on experience the person would have in the real world.